1. Field of the Invention
The present invention relates generally to image reading apparatuses, and more particularly to an image reading apparatus for detecting the size of paper such as an original.
2. Description of the Related Art
As a conventional image reading apparatus of this kind, the techniques disclosed in Japanese Patent Laying-Open No. 62-170948 can be proposed.
According to the above technique, colored light in spectral distribution in a complementary color relationship of color of a pressing plate for pressing an original is directed onto the original to be detected and to the pressing plate. Then, detecting the intensity of reflected light from the original and pressing plate decides whether the reflected light is from the original or the pressing plate so as to detect the presence and size of the original.
As another conventional image reading apparatus of this kind, the technique disclosed in Japanese Patent Publication No. 60-4974 can also be proposed.
In an original exposing device including a platen on which the original is placed and an optical system moving relatively with respect to the platen so as to expose the original on the platen, the above technique comprises a light receiving element on which reflected light from the platen is incident and which provides an output dependent on the intensity of the incident light, comparison means for comparing the output of the light receiving element with a reference value irrespective of the original size, detection means for relatively moving the platen and optical system before exposure of the original to detect a variation of an output of the comparison means at that time, and decision means responsive to a detection signal from the detection means for deciding the size of the original placed on the platen to output a size signal indicating the decided original size. According to this technique, the platen and optical system are relatively moved before the original exposure, so as to detect the variation of the output of the comparison means at that time and decide the size of the original placed on the platen 9 in response to the detection signal thereof.
The former conventional image reading apparatus thus structured premises the use of the colored light in spectral distribution in the complementary color relationship of the color of the pressing plate for pressing the original. Further, this image reading apparatus needs to calculate the size of the original and hence requires a long time for calculation of the original size.
The latter image reading apparatus thus structured decides whether the original is placed on the overall surface of the platen. Therefore, this image reading apparatus needs to decide the state of the overall surface of the platen in order to decide the original size despite the frequent use of an original of normal size in general. Thus, it spends an unnecessarily long time to decide the original size before the original exposure, which is a main object of this apparatus. Consequently, a long time period is required for calculation of the size of the original.
Moreover, recently, a designation of an area to be read is more often carried out for designating a range to be read on the original placed on the platen. Alternatively, such a case occurs more frequently that only a predetermined area is shifted to a designated position in edition mode.
When the area designation of this type is carried out, the detection and calculation of the original size virtually become not only unnecessary operations but also a lost of rising time, becoming a waste of time before the entry to a reading operation of the original such as a copying operation.
Even if it spends a long time to decide the original size, the occurrence of an error in deciding the original size because of a misplacement of the original results in a reading operation to be carried out in accordance with erroneous information, thereby, for example, causing miscopying in case with a copier. That is, even the long time period required for deciding the original size may simply become a waste of time.
In addition, in case of automatic density control by the image reading apparatus of this kind, it is necessary to carry out scanning for the automatic density control after the original size is read by scanning. As described above, in the image reading apparatus in which the decision of the original size and the automatic density control are carried out by scanning, a preliminary scanning time for carrying out such decision and automatic density control becomes unnegligible.
As another conventional image reading apparatus of this kind, the technique disclosed in Japanese Patent Laying-Open No. 60-100157 can be proposed.
This technique comprises size outputting means for outputting original size data, variable scale magnification outputting means for outputting variable scale magnification data, area deciding means responsive to data from the size outputting means and from the variable scale magnification outputting means for deciding a desired image forming area, and area controlling means responsive to data from the area deciding means for controlling a direction of feeding the original and a width direction of the original with respect to the desired area. This technique makes it possible to control an image forming area on a photoreceptor in response to the data from the size outputting means and from the variable scale magnification outputting means.
The above conventional image reading apparatus thus structured needs to evaluate an image forming area in accordance with an output of each of the size outputting means for outputting original size data and the variable scale magnification outputting means for outputting variable scale magnification data. However, the calculation is variable in accordance with the original size data and variable scale magnification data and hence requires a long calculation time period. Further, in case of selecting edition mode or the like to move a predetermined image area, it is possible that the calculation time becomes still increased.
Now, the image reading apparatus disclosed in the above-identified document No. 60-4974, as shown in FIG. 39 showing an output waveform diagram of the foregoing conventional image reading apparatus, relatively moves the platen and the optical system before original exposure, decides the size of the original placed on the platen by the deciding means in response to the detection signal from the detection means for detecting the variation of the output of the comparison means at that time, and outputs a size signal of a rear end detecting output indicating the decided size of the original. At this time, the comparison means compares an output of the light receiving element with a reference value E1 irrespective of the original size. When this reference value E1 is set as a value lower than a white detecting level of the original as a value irrespective of the original size, it is necessary to employ "white" lighter than the white detecting level of the original for an original pressing cover. However, when the white of the original pressing cover lighter than the white detecting level of the original becomes darker, a detection level irrespective of the original size becomes variable, so that the original size may not be satisfactorily detected. If the reference value is set at a white detecting level slightly lighter than the white detecting level of the original in order to cope with the above problem, the image reading apparatus may cause an erroneous operation due to noise or the like upon decision of the kind and size of the original.